1. Field of the Invention
This invention is drawn to a method for vaporizing a viscous material and introducing the material vapor into a CVD reaction apparatus. This invention is also drawn to a process for forming a thin film on a substrate by using the material vaporizing method.
2. Description of the Related Art
Methods for forming thin films incorporating CVD (Chemical Vapor Deposition) techniques are significant in current semiconductor production. Forming thin films using CVD methods sometimes requires the use of a material which is in its liquid phase at normal or room temperature. Typical examples of such materials are: trimethylgallium (TMG), which is used as a source of Ga material in the production of GaAs thin films as a semiconductor compound, and trimethylalumminum (TMA) and dimethylaluminumhydride (DMAH), which are used as sources of Al material in the production of AlGaAs thin films or in the formation of Al thin films which are used as a metal conductor wiring in a semiconductor device.
In order to introduce this kind of liquid material into a CVD reaction vessel, a method has been proposed and used. In this known method, the liquid material is charged in a bubbler. A carrier gas is supplied through a nozzle immersed in the liquid inside the bubbler at a constant rate under the control of a mass-flow controller. This generates bubbles of the carrier gas in the liquid material. The carrier gas bubbles ascend through the liquid while allowing the liquid material to be vaporized into and become mixed with the carrier gas. The material gas thus obtained is introduced into the reaction vessel together with the carrier gas.
An Al-CVD process which uses, for instance, DMAH, uses H.sub.2 gas as the carrier gas, as disclosed in, for example, R. Bhat et al., J. Crystal Growth, vol. 77 pp. 77 (1986).
In this known process, the rate of supply of the carrier gas is on the order of 100 sccm (standard cubic centimeter per minute). In general, mass-production of semiconductors requires the supply of the material at a larger rate. Prior to this invention, however, no practical method has been proposed or used which would realize the required high supply rate when using a highly viscous material. This also makes it difficult to form, at a high speed, a thin film on a semiconductor substrate having a substantial area.
Japanese Patent Laid-Open No. 4-7847 discloses a method in which the pressure at the supply line for the carrier gas is reduced to below the atmospheric pressure, while Japanese Patent Laid-Open No. 4-14827 discloses a method in which a pressure buffering function is provided to act between the vaporizing portion and a reaction vessel. These known methods, however, are intended to achieve a high degree of precision in the material flow rate control, but are not intended to achieve a supply of a highly viscous material at a large rate.